The present invention relates generally to the field of color video projection display systems, and more particularly to techniques for improving contrast in such systems.
In a typical example of a color video projection display system, light from a light source is separated into red, green and blue components. These components in a single panel system are directed sequentially onto a single reflective or transmissive liquid crystal display (LCD) panel which includes a matrix of LCD elements having light reflection or transmission properties that are controllable via signals applied to associated thin-film transistor (TFT) drivers.
An LCD panel modulates the intensity of reflected or transmitted light by changing the polarization of light passing through a liquid crystal layer of the panel. The effect of the LCD panel on the polarization of the reflected or transmitted light depends on factors such as the particular liquid crystal effect used, twist angle, angle of the incident polarization, and retardance. Retardance is also commonly referred to as xe2x80x9coptical path length difference.xe2x80x9d In addition, the polarization change is a function of the voltage applied to the liquid crystal layer. By appropriate variation of the applied voltage, a range of intensity levels including white, black and intermediate gray levels can be generated.
In the above-noted example color video projection display system, the red, green and blue components are scanned across the LCD panel and the LCD elements are controlled in accordance with corresponding red, green and blue signals derived from a video signal, such that each of the components is modulated with its corresponding red, green or blue signal. The resulting modulated components are then directed via a projection lens to a display screen for viewing of the video signal.
More detailed examples of single panel color video projection display systems of the type described above are disclosed in U.S. Pat. No. 5,548,347, issued Aug. 20, 1996 in the name of inventors G. A. Melnik and P. J. Janssen and entitled xe2x80x9cSingle Panel Color Projection Video Display Having Improved Scanning,xe2x80x9d which is incorporated by reference herein. Certain of these systems use a transmissive LCD panel, which is also commonly referred to as a xe2x80x9clight valve.xe2x80x9d Other examples of color video projection display systems are disclosed in U.S. patent application Ser. No. 09/097,969 filed Jun. 16, 1998 and entitled xe2x80x9cProjection Device,xe2x80x9d which is incorporated by reference herein.
A three-panel color video projection display system of the type described above generally includes a separate reflective or transmissive LCD panel for each of the red, green and blue components, with the components being spatially separated such that each component is directed to its corresponding LCD panel. Each of the red, green and blue components is modulated in its corresponding panel by an applied red, green or blue signal derived from a video signal. As in the single panel system, the resulting modulated components are directed via a projection lens to a display screen for viewing of the video signal.
A particular type of reflective LCD panel known as a liquid crystal on silicon (LCoS) display panel uses reflective LCD elements arranged on a silicon backplane. LCoS display panels can be used in both single-panel and three-panel configurations, and are becomingly increasingly popular for use in applications such as compact projectors and head-up or xe2x80x9cnear to the eyexe2x80x9d projection display systems. LCoS display panels have a number of significant advantages over other types of reflective LCD panels. For example, crystalline silicon can be used to form active matrix elements of the LCoS panels. The silicon backplane can also be used to form the TFT drivers and other functional circuitry, using well-known and efficient semiconductor manufacturing techniques. Moreover, a larger percentage of the active area can be used for processing video information for display.
Contrast ratio is an important characteristic of a reflective LCoS display panel or other type of reflective LCD panel. The contrast ratio refers generally to the ratio of a black image to a white image. A technique for improving contrast ratio in a reflective LCD panel is described in J. Gandhi et al., xe2x80x9cPerformance Enhancement of Reflective CMOS TN Displays in Projection Applications Using Compensating Films,xe2x80x9d SID 99 Digest, Paper 47.3, pp. 990-993, SID 1999, which is incorporated by reference herein. This technique uses a combination of two high-retardance films to provide an improvement in contrast ratio in an off-axis projection system having a reflective LCD panel.
A serious problem with the approach of the above-cited reference is that variation between the angles of the high-retardance films yields corresponding variation in the optical axis of the system and in the retardance as a function of wavelength. The two-film arrangement is thus very difficult to optimize. This problem is exacerbated by the high retardance values of the films. Moreover, the use of two films unduly increases the cost and complexity of the off-axis projection system, particularly in a three-panel configuration.
In view of the foregoing, it is apparent that a need exists for improved techniques for enhancing contrast ratio in projection display systems with reflective or transmissive display panels.
The present invention provides a color video projection display system with improved contrast ratio.
In accordance with one aspect of the invention, the contrast ratio is improved through the use of a low-retardance film placed in an optical path of the system either on an input optical path between an input optical path polarizer and a reflective or transmissive panel or on an output optical path between the reflective or transmissive panel and an output optical path polarizer. The low-retardance film may be, e.g., a diacetate film having a retardance of less than about 30 nanometers, with its particular retardance value more specifically being selected to optimize a given system configuration.
The present invention can be implemented in single-panel configurations or multi-panel configurations, e.g., three-panel configurations in which each panel is used for modulating one of red, green or blue incident light.
For example, in one illustrative multi-panel embodiment of the invention, a single low-retardance film is arranged within an input optical path of the system after an input optical path polarizer but at a point prior to separation of the incident light into red, green and blue components for application to the panels.
In another illustrative multi-panel embodiment of the invention, a single low-retardance film is associated with each of the panels, such that the three-panel configuration will include three low-retardance films, each placed in an input or output optical path associated with a corresponding one of the panels. In the case of reflective panels, this may or may not be the same path as in the case of transmissive panels, e.g., the implementation may be on-axis or off-axis, respectively.
The reflective or transmissive panels may be liquid crystal on silicon (LCoS) display panels or other types of reflective or transmissive liquid crystal display (LCD) panels, or any other similar type of light valve devices, as well as various combinations thereof.
Advantageously, the use of a low-retardance film in a color video projection display system configured in accordance with the invention prevents variation in the system optical axis and thus considerably simplifies the optimization process, while also reducing the cost and complexity associated with implementation of the system. Moreover, the low retardance value serves to increase the tolerance of the film angle, further simplifying the implementation.